Composite materials comprising laminated plies of fabric in a resin matrix are often used due to their high strength to weight ratio. Fastening two composite parts together, however, is often troublesome. For example, when one composite part is attached to another composite part, bolts and/or rivets may be used, but such fasteners add weight, increase fabrication cost, and often contribute to local failure modes between the individual plies of the laminate composite.
Composite aircraft stiffeners, typically comprised of carbon matrix material, are used to reinforce thin composite structures such as wing and fuselage skins and bulkhead webs. Other composite constructions, such as ceramic matrix composites (CMCs), provide similar structural support in regions of the aircraft that are subjected to high temperature, such as components associated with engine exhaust, and possibly the engine itself. Due to the frequent mutually perpendicular shape, such stiffeners are often referred to as T-sections. The stiffener attachments must transfer shear loads from the skin to the web portion of the stiffener, as well as out-of-plane loads due to peel (delamination) forces and frame attachments. The web portion of the stiffener extends to a flange portion, which is connected to the skin. Interposed between the web portion and the flange portion is a radius portion, which must transfer structural loads from the stiffener to the skin material. The current practice is to either co-cure, adhesively bond, or mechanically fasten the stiffener to the skin. The co-curing and adhesive bonding techniques offer the minimum weight solution. However, the reliability of co-cured and adhesive bonded joints is generally low. Mechanically fastened joints (e.g. bolts and rivets) have been successfully used, but the use of mechanical joints requires that the laminate be reinforced so that it can react to bolt bearing loads. Additionally, the fasteners themselves are both heavy and expensive and the cost of installation and inspection results in a high cost per fastener.
In addition to bonding techniques, the stiffeners require substantially uniform physical properties to function reliably. The transition between the web portion and the skin of the stiffeners can be problematic, especially for CMCs. Of special concern is a fill-it, which is the region defined by the skin, the radius region(s) and the web portion of the T-section. The fill-it is typically filled with the same matrix material used to separate the composite fibers in the plies. For carbon epoxy composites, this works well, as the matrix material, typically a polymeric composition, is substantially as strong as the fibers. However, the same is not true for CMCs, wherein the matrix material is substantially weaker than the ceramic fibers. Another concern is that the fill-it material for CMCs is exposed to higher fabrication temperatures, such as sintering temperatures wherein ceramic particles are fired to a temperature just below the melting or fusion point to bond the particles together to form a high strength mass. Due to the resulting increase in density of the bonded particles, there is the possibility of separation between the fill-it and the adjacent ceramic plies, creating a structurally weak area.
Instead of the matrix material, CMCs typically use pre-impregnated (“prepreg”) ceramic tows or cloth to form the fill-it. A ceramic fiber tow or cloth is infiltrated with a ceramic-yielding slurry, which prepreg tows are cut and individually built up to form the fill-it. Unfortunately, there is no consistent procedure for inserting the ceramic tows, and the number of prepreg tows that are used in the fill-it is based on art and practice. If too few prepreg tows are used, then the radius portion of the stiffener is too sharp, which result in regions of stress concentration. However, if too many tows are used in the fill-it, then a bulge in the radius may occur, resulting in poor structural properties in the radius portions.
Therefore, what is needed is a composition for use in the fill-it that is inexpensive to make, easy to form and apply in a consistent manner, which provides substantially consistent physical properties in CMC stiffeners.